CLIMATE OF THE SANTA CATALINA MOUNTAINS. 71 
the frost dates which limited the growing season fell between the last 
visit of autumn and the first one of spring. This has particularly been 
the case with all of the lower stations. These circumstances have 
made it necessary to resort to an indirect method of determining the 
dates, which is as follows: A series of graphs was drawn showing the 
march of the weekly absolute minimum temperatures at the Desert 
Laboratory, as registered by thermograph, for the years covered by 
the mountain records. Each reading of minimum temperature for a 
given station was then compared with the minimum for the same 
period at the Desert Laboratory, and the total number of such differ- 
ences was averaged. In this manner it was possible to secure the 
figures given in tables 10 and 11. 
TaBLE 10.—Altitudinal shortening of the frostless season in the Santa Catalina Mountains, 
as shown by the dates of the last spring occurrence and the first autumn occurrence of a 
temperature of 32° at 3 altitudes in 1908, 1909, and 1910. 
Last 32° First 32° 
Station, etc. Year. iv aprnin: ix auton, 
Desert Laboratory; elevation, 2,663 1908 Feb. 20 Nov. 30 
feet; length of frostless season, 285 1909 Mar. 15 Nov. 30 
days. 1910 Feb. 25 Dec. 31 
Averageidates® ...i0cdescusss-wenedardall sani cae Mar. 1 Dec. 10 
5,300 feet; length of frostless season, 1909 Apr. 12 Nov. 10 
Xero-Montane Garden; elevation, 1908 May 10 Oct. 10 
195 days. 1910 Apr. 20 Nov. 26 
AVOTARE CRIES 5 4ccc4 ceeetneaaads Reaslaages exe Apr. 24 Nov. 5 
elevation, 7,600 feet; length of frost- 1909 May 31 Sept. 26 
Montane Garden, Marshall Gulch; 1908 June 15 Sept. 25 
less season, 126 days. 1910 May 9 Oct. 17(?) 
WVOPAGS GALGG. wns bee eee ee es ies esloaeeenas May 29 Oct. 2 
With a knowledge of the average difference between the minimum 
temperature at the Desert Laboratory and at a given station, and with 
the graph showing the march of minima at the Laboratory, it was 
possible to locate the approximate date of the last and first occurrence 
of 32° at the mountain station. Such a graph for 1911 is given in 
figure 17, together with the graph of march of minimum temperatures 
at the Montane Plantation in Marshall Gulch, at 7,600 feet. It will 
be noted that the graph for the Laboratory rises by several pronounced 
jumps during March, April, and May, and falls by precipitate stages 
during September, October, and November. The relatively sudden 
advent of summer and of winter is an invariable annual occurrence, 
and it has helped to make more accurate the estimation of the limiting 
dates for the mountain stations. In the cases in which a minimum 
